[Clang] Do not create dependent CallExpr having UnresolvedLookupExpr inside non-dependent context (#124609)

The `UnresolvedLookupExpr` doesn't get looked up and resolved again
while it is inside the non-dependent context. It propagates into the
codegen phase, causing the assertion failure.

We attempt to determine if the current context is dependent before
moving on with the substitution introduced in the
20a05677f9.

This fixes https://github.com/llvm/llvm-project/issues/122892.

---------

Co-authored-by: Sirraide <aeternalmail@gmail.com>

clang/docs/ReleaseNotes.rst

@@ -355,6 +355,7 @@ Bug Fixes to C++ Support
 - Correctly diagnoses if unresolved using declarations shadows template paramters (#GH129411)
 - Clang was previously coalescing volatile writes to members of volatile base class subobjects.
   The issue has been addressed by propagating qualifiers during derived-to-base conversions in the AST. (#GH127824)
+- Fixed a Clang regression in C++20 mode where unresolved dependent call expressions were created inside non-dependent contexts (#GH122892)
 - Clang now emits the ``-Wunused-variable`` warning when some structured bindings are unused
   and the ``[[maybe_unused]]`` attribute is not applied. (#GH125810)
 

clang/lib/Sema/SemaOverload.cpp

@@ -14361,9 +14361,24 @@ ExprResult Sema::BuildOverloadedCallExpr(Scope *S, Expr *Fn,
     const FunctionDecl *FDecl = Best->Function;
     if (FDecl && FDecl->isTemplateInstantiation() &&
         FDecl->getReturnType()->isUndeducedType()) {
+
+      // Creating dependent CallExpr is not okay if the enclosing context itself
+      // is not dependent. This situation notably arises if a non-dependent
+      // member function calls the later-defined overloaded static function.
+      //
+      // For example, in
+      // class A {
+      //    void c() { callee(1); }
+      //    static auto callee(auto x) { }
+      // };
+      //
+      // Here callee(1) is unresolved at the call site, but is not inside a
+      // dependent context. There will be no further attempt to resolve this
+      // call if it is made dependent.
+
       if (const auto *TP =
               FDecl->getTemplateInstantiationPattern(/*ForDefinition=*/false);
-          TP && TP->willHaveBody()) {
+          TP && TP->willHaveBody() && CurContext->isDependentContext()) {
         return CallExpr::Create(Context, Fn, Args, Context.DependentTy,
                                 VK_PRValue, RParenLoc, CurFPFeatureOverrides());
       }

clang/test/SemaCXX/deduced-return-type-cxx14.cpp

@@ -703,6 +703,48 @@ auto f(auto x) { // cxx14-error {{'auto' not allowed in function prototype}}
   return f(1) + 1;
 }
 
+namespace GH122892 {
+  struct NonTemplate {
+    void caller() {
+        c1(int{}); // since-cxx20-error {{cannot be used before it is defined}}
+        c2(int{}); // since-cxx14-error {{cannot be used before it is defined}}
+    }
+
+    static auto c1(auto x) { // since-cxx20-note {{declared here}} // cxx14-error {{'auto' not allowed in function prototype}}
+    }
+
+    template <typename T>
+    static auto c2(T x) { // since-cxx14-note {{declared here}}
+        return x;
+    }
+  };
+
+  struct FunctionTemplateSpecialized {
+    template <typename T>
+    void specialized(){}
+
+    template <>
+    void specialized<int>() {
+      c1(int{}); // since-cxx20-error {{cannot be used before it is defined}}
+      c2(int{}); // since-cxx14-error {{cannot be used before it is defined}}
+    }
+
+    static auto c1(auto x) { // since-cxx20-note {{declared here}} // cxx14-error {{'auto' not allowed in function prototype}}
+    }
+
+    template <typename T>
+    static auto c2(T x) { // since-cxx14-note {{declared here}}
+        return x;
+    }
+  };
+
+  struct MemberInit {
+    int x1 = c1(int{}); // since-cxx20-error {{cannot be used before it is defined}}
+
+    static auto c1(auto x) { return x; } // since-cxx20-note {{declared here}} // cxx14-error {{'auto' not allowed in function prototype}}
+  };
+
+}
 }
 
 #if __cplusplus >= 202002L